Devices such as fans, internal combustion engines and compressors generate pressure pulses and flow pulses in their exhausts. In particular, with devices such as the internal combustion engines, products of combustion comprise the flow pulses. The pressure and flow pulses are sources of noise, and any restrictions in the flow can create a back pressure that adversely affects the operational efficiency of the device. The equipment that is used to mitigate or reduce the noise level of the pressure and flow pulses in the exhaust gas is commonly referred to as a muffler.
There are many different designs and functional shapes of mufflers. In many cases, the designs of these mufflers create substantial back pressure or do not adequately reduce the sounds that are emitted from an exit such as a tail pipe.
Exhaust noise can be characterized at the outlet by acoustic spectra with combinations of tones and broadband sound pressure levels. It is desirable for a muffler to reduce the noise over a wide range of frequencies with minimal flow blockages in the duct or pipe. Automobile exhausts are one such application where mufflers are used to suppress noise from an engine. Many of the “stock” mufflers provide sufficient noise reduction at the expense of increasing the exhaust back pressure due to flow blockages in the muffler. There are also “straight through” muffler designs that reduce the flow losses, but they have limited noise reduction capability.
A resonator muffler partially accomplishes this sound reduction objective. Helmholtz resonators in the form of side branches and tanks have been used to introduce impedance changes to reduce the overall sound from exhaust systems. Another approach is to use an expansion chamber. It is well established that the area ratio from the sudden expansion controls the amplitude of the noise reduction and the length determines the resonant frequency of the chamber. Expansion chambers provide a wider frequency range of noise reduction compared to side branch and tank mufflers. A schematic of a prior art expansion chamber is shown in FIG. 1. Pictured is a muffler 10 having an inlet 12 and an outlet 14 joined by a through passage pipe 16. The inlet passes through end enclosure 26 and the outlet passes through an outlet end enclosure 28. A plurality of perforations 18 allow gases to pass radially outward through the pipe 16 into an expansion chamber 22. The outer diameter of the expansion chamber is defined by an outer shell 24 forming a confined space with end enclosures 26, 28. The center perforated through pipe 16 is used to reduce the flow losses, but can be removed to make a classical expansion chamber. Resonator mufflers have been shown to work well for low frequencies as long as there is enough space since they can be large in size. However, they often introduce a low frequency system resonance due to acoustic interaction with the tailpipe.
Another type of muffler in common use is called a “dissipative” muffler. Sound absorbing material is used to convert the sound pressure into heat that reduces the amplitude of pressure waves over a range of frequencies. Dissipative mufflers are useful for absorbing sound at frequencies above about 400 Hz. A schematic of a typical dissipative muffler is shown in FIG. 2. As previously described, the muffler 10 has an inlet 12 and an outlet 14 joined by through passage pipe 16. The inlet extends through end enclosure 26 and the outlet extends through end enclosure 28. A plurality of perforations 18 in the pipe 16 allow gases to pass radially outward through the passage 16 into a chamber 30 filled with a suitable dissipative material 20. The outer diameter of the dissipation chamber is defined by an outer shell 24.